14.5 and 14.6 Bootgen has been improved to handle security (AES and RSA).
The -split option needs to be re-implemented and it is currently not included in 14.5 and 14.6 Bootgen.
The workaround is to use:
If you need the -split option in a secure flow, please contact Xilinx support.
Xilinx Engineering is working to reinstate the -split option.